1. Field of the Invention
The present invention relates to a modulation sensitivity correction circuit for a voltage-controlled oscillator, and particularly relates to a modulation sensitivity correction circuit for substantially equalizing modulation sensitivity of a voltage-controlled oscillator in a frequency synthesizer used for land mobile telephone or the like.
2. Description of the Prior Art
FIG. 3 shows a conventional phase-locked loop (PLL) synthesizer which is provided with: a control section 1 for converting channel data received at a data input terminal IN.sub.1 into a channel signal; a level adjuster 2 for stepwise changing the level of a modulation signal for frequency modulation or the like such as an audio signal applied through another input IN.sub.2 on the basis of the channel signal from the control section 1; a voltage-controlled oscillator (VCO) 3; a variable frequency divider 4 for dividing the oscillation frequency F.sub.0 of the VCO 3 into a predetermined frequency on the basis of the data from the data input terminal IN.sub.1 ; a phase detector (PD) 5; a temperature-compensated crystal oscillator (TCXO) 6 which acts as a highly stable reference oscillator; a fixed frequency divider 7 for dividing the oscillation frequency F.sub.s of the TCXO 6 into a predetermined frequency; a low pass filter (LPF) 8 for eliminating useless noises; and an output terminal OUT.sub.1 for the VCO 3.
In the thus arranged circuit, the oscillation frequency F.sub.0 of the VCO 3 is divided by the variable frequency divider 4 into a predetermined frequency which is in turn applied to the PD 5 on one hand, and the oscillation frequency F.sub.s of the TCXO 6 is divided by the fixed frequency divider 7 into a predetermined frequency which is in turn applied also to the PD 5 on the other hand. The respective output signals of the variable frequency divider 4 and the fixed frequency divider 7 are phase-detected by the PD 5. The output V.sub.PD of the PD 5 is fed back to the VCO 3 as a control voltage V.sub.F for the VCO 3 after elimination of useless noise in the LPF 8.
FIG. 4A shows the relation between the control voltage or signal V.sub.F applied from the LPF 8 to the VCO 3 and the oscillation frequency F.sub.0 of the VCO 3, and FIG. 4B shows the relation of the modulation sensitivity Sm to the control signal V.sub.F. The modulation sensitivity Sm changes if the oscillation frequency F.sub.0 is changed on the basis of control signal V.sub.F. Accordingly, if the voltage applied to the VCO 3 is changed in four steps by the level adjuster 2, the modulation sensitivity has four discontinuous characteristics with respective gradients which decrease as the channel number or frequency increases, as shown in FIG. 4C.
In such an arrangement, however, in order to accurately control the allowable fluctuation range of the transmission modulation sensitivity of a multi-channel radio equipment having 600 channels, 1000 channels, or the like, it is necessary to divide the channel of change-over of the modulation sensitivity into many steps, for example 6 steps, 10 steps, or the like. In that case, there is a problem in that it becomes necessary to provide a complicated control circuit including number of decoders.